Theoretical Study Of Transport Research Articles

Nonreciprocal superconducting transport and the spin Hall effect in gyrotropic structures

The search for superconducting systems exhibiting nonreciprocal transport and, specifically, the diode effect, has proliferated in recent years. This trend has encompassed a wide variety of systems, including planar hybrid structures, asymmetric SQUIDs, and certain noncentrosymmetric superconductors. A common feature of such systems is a gyrotropic symmetry, realized on different scales and characterized by a polar vector. Alongside time-reversal symmetry breaking, the presence of a polar axis allows for magnetoelectric effects, which, when combined with proximity-induced superconductivity, results in spontaneous non-dissipative currents that underpin the superconducting diode effect. With this symmetry established, we present a comprehensive theoretical study of transport in a lateral Josephson junction composed of a normal metal supporting the spin Hall effect, and attached to a ferromagnetic insulator. Due to the presence of the latter, magnetoelectric effects arise without requiring external magnetic fields. We determine the dependence of the anomalous currents on the spin relaxation length and the transport parameters commonly used in spintronics to characterize the interface between the metal and the ferromagnetic insulator. Therefore, our theory naturally unifies nonreciprocal transport in superconducting systems with classical spintronic effects, such as the spin Hall effect, spin galvanic effect, and spin Hall magnetoresistance. We propose an experiment involving measurements of magnetoresistance in the normal state and nonreciprocal transport in the superconducting state. Such experiment would, on the one hand, allow for determining the parameters of the model and thus verifying with a greater precision the theories of magnetoelectric effects in normal systems. On the other hand, it would contribute to a deeper understanding of the underlying microscopic origins that determine these parameters.

Theoretical study of transport of MHD peristaltic flow of fluid under the impact of viscous dissipation

ABSTRACT This article studies irreversibility analysis in the peristaltic transport of a non-Newtonian fluid. Jeffrey fluid is treated as the blood transport through a flexible artery/vessel. A peristaltic wave travels on the porous opposite wall of the inclined channel under the magnetic environment. Heating effects cause the shear thinning in the viscosity of the base liquid, transferring tiny particles. The efficiency of peristalsis is improved by reducing the irreversibility factor by means of entropy generation. The perturbation technique is applied to understand the nonlinear flow dynamics of heat and mass transfer with Jeffrey fluid with the help of convective boundary conditions. Comparative analysis is carried out with the available literature for the limiting case and found to have complete coherence. A detailed parametric study is carried out to comprehend the contribution of significant parameters which reveals that magnetic fields and slippery walls induce a resistive force on the nanoflow. The motion of the base liquid expedites subject to variation in Darcy number and shear-thinning effects. The energy is contributed to the nanofluid system in response to the Brinkman number. More, the number density of nanospecies reduces in the region of higher concentration subject to the Soret number and Schmidt number, respectively.

Experimental and theoretical studies of transport and optical properties of binary mixtures of acetonitrile with some alkyl methacrylates at temperatures from 298.15 to 318.15 K

The viscosities, η and refractive indices, nD of pure acetonitrile, methyl methacrylate, ethyl methacrylate, butyl methacrylate and of their binary mixtures with acetonitrile as the common component, covering the entire composition range were measured at temperatures (298.15, 303.15, 308.15, 313.15 and 318.15) K and atmospheric pressure. Using these experimental data, the deviations in viscosity ∆η, deviations in refractive index, ∆nD , deviations in molar refraction, ∆Rm have been calculated. These excess properties were correlated by the Redlich-Kister polynomial equation. The variations of ∆η, ∆nD and ∆Rm with composition and temperature has been discussed terms of intermolecular interactions existing in these mixtures. Further, the viscosities and refractive indices of these binary mixtures were calculated theoretically by using various empirical and semi-empirical relations and the results were compared with the experimental findings.

Coherent transport in Y-junction graphene waveguide

We performed a series of theoretical transport studies on Y-branch electron waveguides which are embedded in mid-size armchair graphene nanoribbons. Non-equilibrium Green’s function with different approximations of tight-binding Hamiltonian has been employed. Using the first nearest hopping approximation, we observed very pronounced conductance quantization, the structure of which depends on geometrical design and shows a spacing of 4e2/h, indicating the existence of valley degree of freedom. Moreover, by incorporating the third nearest approximation, we observed seminal plateaus deviated from multiples of 4e2/h conductance, suggesting the lift of valley degeneracy. Finally, Quasi-one dimensional band structure calculations have been performed to study the availability of energy channels and the role of the major geometrical parameters on the transport.

Role of Edge Engineering in Photoconductivity of Graphene Nanoribbons.

The effect of edge engineering of graphene nanoribbons (GNRs) on their ultrafast photoconductivity is investigated. Three different GNRs were fabricated by bottom-up synthesis in the liquid phase, where structure, width, and edge planarity could be controlled chemically at the atomic level. The charge carrier transport in the fabricated GNRs was studied on the ultrafast, sub-picosecond time scale using time-resolved terahertz spectroscopy, giving access to the elementary parameters of carrier conduction. While the variation of the side chains does not alter the photoconductive properties of GNRs, the edge structure has a strong impact on the carrier mobility in GNRs by affecting the carrier momentum scattering rate. Calculations of the ribbon electronic structure and theoretical transport studies show that phonon scattering plays a significant role in microscopic conduction in GNRs with different edge structures. A comparison between theory and experiment indicates that the mean free path of charge carriers in the nanoribbons amounts to typically ∼20 nm.

Manifestation of the shape and edge effects in spin-resolved transport through graphene quantum dots

We report on theoretical studies of transport through graphene quantum dots weakly coupled to external ferromagnetic leads. The calculations are performed by exact diagonalization of a tight-binding Hamiltonian with finite Coulomb correlations for graphene sheet and by using the real-time diagrammatic technique in the sequential and cotunneling regimes. The emphasis is put on the role of graphene flake shape and spontaneous edge magnetization in transport characteristics, such as the differential conductance, tunneling magnetoresistance (TMR) and the shot noise. It is shown that for certain shapes of the graphene dots a negative differential conductance and nontrivial behavior of the TMR effect can occur.

Perfect Reflection of Chiral Fermions in Gated Graphene Nanoribbons

We describe the results of a theoretical study of transport through gated metallic graphene nanoribbons using a nonequilibrium Green function method. Although analogies with quantum field theory predict perfect transmission of chiral fermions through gated regions in one dimension, we find perfect reflection of chiral fermions in armchair ribbons for specific configurations of the gate. This effect should be measurable in narrow graphene constrictions gated by a charged carbon nanotube.

Experimental and Theoretical Studies of Transport through Large Scale, Partially Aligned Arrays of Single-Walled Carbon Nanotubes in Thin Film Type Transistors

Gate-modulated transport through partially aligned films of single-walled carbon nanotubes (SWNTs) in thin film type transistor structures are studied experimentally and theoretically. Measurements are reported on SWNTs grown by chemical vapor deposition with systematically varying degrees of alignment and coverage in transistors with a range of channel lengths and orientations perpendicular and parallel to the direction of alignment. A first principles stick-percolation-based transport model provides a simple, yet quantitative framework to interpret the sometimes counterintuitive transport parameters measured in these devices. The results highlight, for example, the dramatic influence of small degrees of SWNT misalignment on transistor performance and imply that coverage and alignment are correlated phenomena and therefore should be simultaneously optimized. The transport characteristics reflect heterogeneity in the underlying anisotropic metal-semiconductor stick-percolating network and cannot be reproduced by classical transport models.

Thermoelectric efficiency of single crystal semiconducting ruthenium silicide

Thermoelectric efficiency of semiconducting ruthenium silicide Ru2Si3 has been systematically studied both experimentally and theoretically. Pure and Mn-doped Ru2Si3 single crystals were grown by zone melting with optical heating. Temperature dependences of the resistivity, Hall factor, Seebeck coefficient, and thermal conductivity were studied in the range of 100–900 K. For Mn-doped Ru2Si3 crystals, the Seebeck coefficient is positive in the whole temperature range under study, it reaches its maximum value of 400 μV/K at about 500 K. At room temperature, the Seebeck coefficient of these crystals is about 300 μV/K, which is twice as high as in the undoped material. The theoretical study of transport and thermoelectric properties includes the ab initio calculation of band structure, estimation of the carrier effective masses, modeling of the electron and hole mobilities in terms of classical scattering mechanisms, and calculation of the Seebeck coefficient and thermoelectric figure of merit, ZT. The results of theoretical modeling show a good qualitative and quantitative agreement with the experimental data.

Interplay of Kondo and Fano Resonance in Electronic Transport in Nanostructures

Quantum interference and electronic correlations are processes relevant for electronic transport in nanostructures. We present theoretical studies of transport for various models, which take into account these processes and describe the Fano resonance as well as many-body effects like the Kondo resonance. The results are compared with transport measurements performed on a small quantum dot strongly coupled to electrodes, a large semi-open quantum dot, a side-attached quantum dot to a quantum wire systems and a quantum dot embedded in the metallic ring. We show how conductance characteristics of the Kondo and the Fano resonance are modified in different situations.

A Parallel Electromagnetic Molecular Logic Gate

A theoretical study of transport through a three-terminal molecular ring in the presence of a magnetic field is presented. The physical principles necessary to achieve logic operations based on the Aharonov-Bohm effect are discussed. We show that a proper combination of a gate potential and a realistic magnetic field can be used to obtain parallel logic operations such as AND and AND+NOT.

Spatiotemporal carrier dynamics in quantum wells under surface acoustic waves

We present a theoretical study of transport and recombination of electrons and holes in quantum wells under the piezoelectric field induced by a surface acoustic wave (SAW). Our model calculations, which include free carriers and excitons in the framework of the drift-diffusion equations, describe the spatial and time dependences of the photoluminescence intensity on excitation density and SAW amplitude, and show overall agreement with recent microphotoluminescence experiments performed on GaAs/(Al,Ga)As quantum wells and quantum wires.

Proximity-induced transport in hybrid mesoscopic normal–superconducting metal structures

Using an approach based on quasiclassical Green’s functions we present a theoretical study of transport in mesoscopic SN structures in the diffusive limit. The subgap conductance in SN structures with barriers (zero bias and finite bias anomalies) are discussed. We also analyse the temperature dependence of the conductance variationδS (T) for a Andreev interferometer. We show that besides the well know low temperature maximum a second maximum near Tcmay appear. We present the results of studies on the Josephson effect in 4-terminal SNS contacts and on the possible sign reversal of the Josephson critical current.

Experimental and theoretical study of ultra-thin oxides

We report on an experimental and theoretical study of transport through thin oxides. The experimental study was carried out on the tunnel switch diode (TSD) which consists of an MOS junction on top of a pn junction. The properties of the TSD depends critically on the properties of the tunnel oxide layer. Our results indicate that these devices can exhibit two different modes of behaviour depending on the stress history of the oxide. An unstressed device exhibits a thyristor-like I-V characteristic with fairly low current density. As the oxide is stressed, however, the I-V characteristic discontinuously shifts into a higher-current thyristor-like mode in which current transport appears to be highly non-uniform and depends strongly on stress history. This suggests a possible structural change in the oxide layer which is not completely destructive in that the device continues to function. We present a possible theoretical model of such a structural change in which microscopic filaments are generated in the oxide. Calculations of J-V curves for such structures with varying filament heights qualitatively match stressed MOS I-V curves found in the literature and qualitatively explain the dual-mode behaviour of the TSD.

Theoretical studies of transport of solid particles in pulsating flows

The dynamics of the motion of solid particles in flows with a pulsation frequency of the flow rate of up to 1 Hz has been studied. Diagrams characterizing the contributions of the various components to the resultant force exerted by the fluid on the particles have been obtained.

Theoretical study of transport through a quantum point contact.

We developed a formalism within the linear-response theory to investigate the transport through a quantum point contact between two electron-gas reservoirs. It is valid for two-terminal conductance through a constriction of a two-dimensional (2D) or 3D potential and has a wide range of applicability covering ballistic as well as tunneling regimes. We studied the quantization of conductance and examined several effects influencing the quantum transmission. Among these effects we found that the simple phase relation results in resonance structures superimposed on the plateaus between two steps of quantized conductance. These resonances are destroyed by the smooth entrance, finite temperature and bias, and variation of the potential. The simulation of adiabatic transmission in constrictions having smoothly varying widths resulted in the conductance with sharp quantum steps without the resonance structure. The quality of quantization is strongly affected by the length of constriction, Fermi-level smearing, the obstacle at the entrance, impurity scattering, nonuniformities of geometry and potential, and in particular by the variation of the longitudinal potential resulting in a sharp saddle-point structure.

Electronic excited-state transport and trapping in solution

A theoretical study of transport and trapping of electronic excitations in a two-component disordered system is carried out. The results are applicable to energy transport in solutions containing randomly distributed donor and trap solute species or lattices with randomly distributed donor and trap impurities. The diagrammatic expansion of the Green function, developed by Gochanour, Andersen, and Fayer to study excited-state energy transport in a one-component system is applied to the trapping problem. The following quantities are calculated from the Green function: the time-dependent probabilities that an excitation is in the donor or in the trap ensembles, the generalized diffusion coefficient, and the mean-squared displacement of an excitation. For Förster transfer, transport properties are shown to depend on the ratio of the Förster interaction lengths RDT0 and RDD0 as well as on the reduced concentrations of donors and traps [CD = 4/3π(RDD0)3ρD, CT = 4/3π (RDT0)3ρT]. The tranport of excitations is found to be nondiffusive. A comparison to other theoretical treatments is presented.